U.S. Pat. No. 4,207,251 discloses the gas phase production of C.sub.1 -C.sub.24 organo-isocyanates by oxidative dehydrogenation of the corresponding N-monosubstituted formamides. This reaction is extremely exothermic, and consequently some method of temperature control or heat removal during the reaction has been employed to maintain the reaction temperature below that which would accelerate side reactions or actual decomposition of the isocyanate product. U.S. Pat. No. 4,207,251, for example, discloses the use of small diameter reactors or the introduction of relatively large volumes of an inert gas to the reaction mixture to absorb the heat liberated during the reaction. Disadvantages attend both of these methods, however. Small-diameter reactors that are designed for cooling are expensive and have relatively high surface-area/volume ratios, which often necessitates the use of several reactors in parallel, an arrangement which is difficult to operate. The use of an inert heat sink at the high volumes disclosed in the patent (over 50 volumes of inert gas per volume of oxygen), presents difficulty and expense in separation of the desired product. The improvement of the present invention eliminates the need for active cooling of the reactors and permits the reduction in volume, or entire elimination, of the inert gas.